This invention relates to the measurement of chemical entities in liquid samples.
A wide variety of analytical systems are available for measuring chemical entities such as ions, e.g., potassium, sodium, and chloride; gases, e.g., O.sub.2 ; and organic compounds, e.g., glucose, in liquid samples such as blood, urine, and liquids related to industrial processes. Such systems contain components which from time to time require maintenance or replacement.